Abstract Atypical speech rhythms, like those observed in children with childhood apraxia of speech, autism spectrum disorders, and stuttering, limit social interactions and can impede normal development and cognitive functioning. The long-term goal of the proposed research is to provide a developmentally sensitive model of language production that explains the emergence of typical and atypical speech rhythms. Current models of production are based on sophisticated descriptions of adult language. Influences from slowly developing speech motor, language, and other cognitive skills on speech plan representations and production processes have rarely been investigated. The paucity of research on such influences leads to fundamental gaps in understanding typical and atypical rhythm development, including understanding why young school-aged children do not chunk grammatical words with content words in connected speech to the same extent as adults. This chunking is important because it yields the rhythms we associate with prosodic words in adult language. The proposed research will investigate age-related changes in grammatical word production in typical development to explain prosodic word acquisition. The working hypothesis is that, once lexical stress is acquired, adult-like English rhythm production depends on a meaning- based chunking process at the intonational phrase level. The chunks evolve over developmental time into the supralexical production units we hear as prosodic words. We will test this hypothesis against well-defined alternatives using long-distance coarticulation to index production units. English-speaking school-aged children (5 and 8 years old) and adults will produce sentences designed to elicit either anticipatory or perseveratory lip rounding at different distances from a target content-word vowel. Metrical contexts will also be varied, and children's linguistic and cognitive development independently assessed. An audio-visual gating paradigm will be used to test for coarticulatory onset/offset at specific phonological and temporal locations. Perceiver agreement and acoustic measures will be used to gauge coarticulatory strength. The influence of specific linguistic and cognitive factors on the development of long-distance coarticulation will also be investigated. In all, the proposed research will accomplish 3 specific practical aims. It will (1) identify the scope of ?look ahead? in children's speech and factors influencing the development of anticipatory coarticulation across word junctures; (2) measure the strength of perseveratory and anticipatory coarticulation on grammatical word production across different metrical contexts and ages; and (3) assess the effect of long-distance coarticulation versus other factors on the perceptual coherence of grammatical-content word sequences. The results will inform theory and provide new insights into why speech rhythm is atypical in children with speech planning deficits. The research will also provide important normative data on the extent and strength of long-distance coarticulation in school-aged children's speech, and fundamental information about language and speech interactions in typical development. Finally, it will determine which cues most influence the perception of immature speech rhythm, with implications for clinical intervention.